As an apparatus for driving a rotary machine such as a compressor, there is an apparatus including an electric driving device for generating a rotational driving force and a transmission device for changing the speed of a rotational driving force generated by the electric driving device and then transmitting the speed-changed rotational driving force to the rotary machine.
Patent Document 1 discloses that a constant-speed motor and a variable-speed motor for speed change are used as the electric driving device and a planetary gear transmission device is used as the transmission device to accurately control a gear ratio. In this apparatus, it is possible to change a rotation rate of an output shaft of the transmission device connected to the rotary machine by changing a rotation rate of the variable-speed motor. The rotation rate of the variable-speed motor can be changed by changing a frequency of an AC power source using an inverter.